The Army's special branch: How bizarre fake spy trees appeared in no-man's land during WWI (and killed hundreds of soldiers)
Daily Mail
Artists in the Royal Engineers were tasked with meticulously selecting a real tree on the battlefield by measuring and photographing it extensively. The ideal tree was dead and often it was bomb blasted. The photographs and sketches were then sent to a workshop where artists constructed an artificial tree of hollow steel cylinders. It contained an internal scaffolding for reinforcement, to allow a sniper or observer to ascend within the structure. Then, under the cover of night, the team cut down the authentic tree and dug a hole in the place of its roots, in which they placed the O.P. Tree. When the sun rose over the battlefield, what looked like a tree was a tree no longer. Instead, it was an enemy lookout tower.